1. Field of the Invention
The present invention relates generally to an optical isolator and, more particularly, to a manufacturing method capable of highly efficiently mass-producing small, lightweight optical isolators having stable characteristics, to a polarizer array for use in the above manufacturing method, and to an optical module incorporating an optical isolator manufactured by the above method.
2. Description of the Prior Art
It is known that a semiconductor laser becomes unstable in its performance when exposed to light reflected by the surface of an optical part such as a condenser lens or an optical fiber cable connected behind the condenser lens. Such a disadvantage of the semiconductor laser is a significant problem in the reliability of optical systems in the field of optical communications and optical instrumentation. An optical isolator may be an effective means for solving such a problem.
Basically, an optical isolator comprises polarizers and a Faraday rotator as typically illustrated in FIG. 1. The optical isolator shown in FIG. 1 comprises a pair of polarizers 1, and a Faraday rotator 2 disposed between the polarizers 1. The polarizers 1 are birefringent prisms or polarizing beam splitters, and the Faraday rotator 2 is a magnetic garnet crystal which is surrounded by an annular permanent magnet 3 for applying a magnetic field thereto. The polarizers 1, the Faraday rotator 2 and the permanent magnet 3 are contained in a case 4.
The optical isolator thus constructed, however, has disadvantages that it has a large size with considerable weight since the components, particularly the polarizers 1, are large.
The inventors herein proposed a novel optical isolator capable of solving the foregoing problems and having structures as disclosed in Japanese Patent Publication No. 61-16961. As shown in FIGS. 2(A)-2(C), this optical isolator comprises a pair of polarizers 5 and 6, a Faraday rotator 7 and a permanent magnet 8. The polarizer 5 is formed by alternately superposing transparent dielectric films 5a of several thousands of angstroms in thickness and films 5b having complex a dielectric constant, formed of metal or semiconductive material and having a thickness of several tens of anstroms (FIG. 2(A)), and the polarizer 6 is similarly formed by alternately superposing transparent dielectric films 6a and films 6b having a complex dielectric constant (FIG. 2(C)). These polarizers 5 and 6, when compared with those shown in FIG. 1, are very small and very lightweight. However, a method of manufacturing the optical isolator of such a construction has the following technical problems.
In manufacturing the optical isolator shown in FIG. 2(B) the minute polarizers 5 and 6 have hitherto been applied to opposite end surfaces of the Faraday rotator 7. Therefore, skilled labor and much time are required for assembling the optical isolator, and hence this manufacturing method is not suitable for mass production, and optical isolators manufactured by this method ar subject to large quality variations.
Furthermore, in assembling the optical isolator shown in FIG. 2(B), the polarizers 5 and 6 must be applied to the corresponding end surfaces of the Faraday rotator 7 with the layers of the films 6b of the polarizer 6 turned accurately by an angle of 45.degree. relative to the layers of the films 5b of the polarizer 5. It is a difficult assembly operation to position the polarizer 6 relative to the polarizer 5 because of their minute size, resulting in large variations in characteristics of the optical isolators thus obtained.
The optical isolator has often been combined with a semiconductor laser etc. to form an optical module, an example thereof being illustrated in FIG. 3. This optical module comprises a semiconductor laser a, an optical fiber cable b having the same optical axis as that of the semiconductor laser a, a Faraday rotator d, a permanent magnet c surrounding the Faraday rotator d, a lens e for controlling a laser beam, a polarizer f and a case g accommodating those components. The Faraday rotator d, the lens e and the polarizer f are arranged between the semiconductor laser a and the optical fiber cable b.
This optical module is not provided with any polarizer between the semiconductor laser a and the Faraday rotator d. Although this structure satisfies the requirements in a certain range, it is desirable to replace the Faraday rotator d and polarizer f with an optical isolator of the type having a pair of polarizers. The optical isolator disclosed in the above Japanese Patent Publication would make it possible to miniaturize and reduce the weight of the optical module to a considerable extent. However, as mentioned above, the method of manufacturing the optical isolator of FIG. 2(B) has the problem that it is not possible to mass-produce optical isolators having stable characteristics. Particularly, once an optical module is assembled, it is impossible to change the optical isolator incorporated therein, and hence, the optical isolator must undergo strict inspection, thus causing its price to be very expensive.
Furthermore, since the components of the optical module in FIG. 3 are arranged at intervals in alignment with an optical axis, a simple replacement of the Faraday rotator d and the polarizer f with the optical isolator in FIG. 2(B) will require troublesome alignment of the optical axes of the components.